To become a mediator, you must attend a mediation training course.
The following organisations offer CMC Registered training courses which can lead to CMC Registered status (see here [link] for all of the registration requirements).
Academy of Experts
Align Mediation (though not the distance learning course)
Globis Mediation Group
London School of Mediation
M & D Associates
National Dispute Resolution
Phoenix Dispute Solutions
Royal Institute of Chartered Surveyors (RICS)
Society of Mediators
University of Central Lancashire